1. Field of the Invention
The invention is useful in the field of radio-frequency (RF) transmission, and in particular to digital demodulation of quadrature phase amplitude modulated RF signals.
2. Discussion of Background Information
Certain techniques for amplitude modulation and demodulation of a radio frequency carrier are known. FIG. 1 illustrates a demodulation method for an amplitude modulated (AM) signal using digital down converters. An incoming amplitude modulated RF 10 signal is filtered 12 and quantized 14 to form a quantized AM signal 16. The quantized AM signal 16 is downconverted 18 to form an in-phase baseband signal 20 which may be low-pass filtered 22. The quantized AM signal 16 is also downconverted 24 to form a quadrature-phase baseband signal 26 which may be low-pass filtered 28. A phase lock loop (PLL) circuit uses a numerically controlled oscillator (NCO) 34 to generate a local carrier signal 36 that tracks the carrier phase of quantized AM signal 16. The local carrier signal 36 is used to downconvert the quantized AM signal 16 into the in-phase baseband signal 20 using a well-known multiplication relationship. The carrier signal 36 is also phase shifted 38 and used to downconvert the AM signal 16 into the quadrature-phase baseband signal 26 using a well-known multiplication relationship. One baseband signal, such as a quadrature-phase baseband signal 30, is feed directly to a PLL filter 32 for control of the NCO 34. Conventional AM demodulation methods assume that the pass band of the PLL filter is larger than the highest frequency signal of interest.
The demodulation method illustrated in FIG. 1 relies on a well-known algebraic approximation that sin (x)≈x for small x. In the method of FIG. 1, the NCO would ideally be controlled by the sine of the error between the NCO-generated carrier phase 36 and the carrier phase of the quantized AM signal 16. For simplification, the estimation signal 30 used to control the NCO is the baseband signal. Under this approximation, the quadrature phase baseband signal goes to zero when the detector is ideally locked. Additional background information on phase lock loops AM demodulation can be found in H. Meyr, Synchronization in Digital Communications & Digital Communications Receivers, John Wiley & Sons, Inc. New York, 1998.